Strong bonded joints for cryogenic application

ABSTRACT

A method and apparatus for strong bonded wide joints for cryogenic applications. In one advantageous embodiment, an apparatus may comprise a three-dimensional preform and a plastic matrix. The plastic matrix may be impregnated in the three-dimensional preform to form a softening strip that may be capable of remaining flexible at a temperature at which a gas may have a liquid form.

BACKGROUND INFORMATION

1. Field

The present disclosure relates generally to manufacturing vehicles andin particular to manufacturing spacecraft. Still more particularly, thepresent disclosure relates to a method and apparatus for manufacturingbonded joints for low temperature applications.

2. Background

A spacecraft may be a vehicle designed for space flight. Spacecraft maybe used for various purposes, such as, for example, without limitation,communications, earth observation, meteorology, navigation, and othersuitable functions. Spacecraft may be reusable. For example, withoutlimitation, a space shuttle is an example of a reusable spacecraft. Alaunch vehicle may be another form of a spacecraft used to lift anotherspacecraft into orbit or space.

Both a space shuttle and a launch vehicle may have propulsion systems.These propulsion systems may include various types of propellants suchas, for example, without limitation, a solid propellant, a liquidpropellant, a mixture of both solid and liquid propellant, and/or anyother suitable propellant. Liquid propellants may include, for example,without limitation, liquid hydrogen and/or liquid oxygen. These gasesmay be stored in liquid form in cryogenic tanks. A cryogenic tank maystore and/or maintain liquid propellants at very low temperatures, suchas, for example, without limitation, below −238 degrees Fahrenheit.

In a spacecraft or launch vehicle that uses these types of propellants,these tanks may be connected to the structure of the spacecraft usingjoints. A joint is a location and/or place where a first structure maybe attached to a second structure. Y-joints may be used to connect thetanks to the spacecraft structures. It may be desirable to increase thestrength of lap joints above current levels. A Y-joint with a softeningstrip may be used to provide the needed strength.

These types of joints, however, may be subjected to loading that exceedsthe strength of simple lap joints. With the use of cryogenicpropellants, the joints used to attach the tanks to the spacecraftstructure may be subjected to very cold temperatures. Most materials foruse as a softening strip may become too stiff at cryogenic temperaturesand/or may have a coefficient of thermal expansion that may beincompatible with other joint components. A softening strip may be acomponent used to reinforce a joint.

Therefore, it would be advantageous to have a method and apparatus thatovercomes the problems discussed above.

SUMMARY

The advantageous embodiments may provide a method and apparatus forstrong bonded joints for cryogenic applications. In one advantageousembodiment, an apparatus may comprise a three-dimensional preform and aplastic matrix. The plastic matrix may be impregnated in thethree-dimensional preform to form a softening strip that may be capableof remaining flexible at a temperature at which a gas may have a liquidform. In another advantageous embodiment, a method may be present forforming a joint. A softening strip may be placed into a joint region fora first structure, wherein the softening strip is capable of remainingflexible at a temperature at which a gas has a liquid form. A secondstructure may be laid up. The softening strip may be bonded to the firststructure and to the second structure to form the joint.

In yet another advantageous embodiment, a method may be present formanufacturing a softening strip. A three-dimensional preform may beformed. A plastic material may be infused into the three-dimensionalpreform to form a composite material capable of remaining flexible at atemperature at which a gas may have a liquid form.

In still yet another advantageous embodiment, a launch vehicle maycomprise a structure, a tank, and a softening strip. The structure has afirst side, wherein the structure may be comprised of a materialselected from at least of a metal, a metal alloy, and a compositematerial. The tank may be capable of holding a liquid propellant and mayhave a first side bonded to the first side of the structure. The tankmay be comprised of a material selected from one of a metal, a metalalloy, and a composite material.

The softening strip may have a three-dimensional preform and a plasticmatrix impregnated in the three-dimensional preform to form thesoftening strip that may be capable of remaining flexible at atemperature at which a gas may have a liquid form. The three-dimensionalpreform may be a three-dimensional fabric comprising at least one ofweaved fibers, braided fibers, and stacked fabric layers in which thefibers are selected from one of graphite fibers, glass fibers, andaramid fibers. The plastic matrix may be selected from at least one of afluorocarbon and a urethane. The softening strip may be bonded to thefirst side of the first structure and the first side of the tank to forma Y-joint.

In a further advantageous embodiment, a method may be present forforming a joint in a spacecraft. A three-dimensional fabric preform maybe formed. A plastic material may be infused into the three-dimensionalfabric preform to form a plastic matrix to form a softening strip. Thesoftening strip may be capable of remaining flexible at a temperature inwhich gas may have a liquid form. The three-dimensional fabric preformmay be comprised of at least one of weaved fibers, braided fibers, andstacked fabric layers in which the fibers are graphite fibers, andwherein the plastic material is selected from at least one of afluorocarbon and a urethane. A first adhesive layer may be attached to afirst side of the softening strip. A second adhesive layer may beattached to a second side of the softening strip. A first surface of aspacecraft structure for the spacecraft may be positioned relative to asecond surface of a second structure. The softening strip may be placedbetween a portion of the first surface and a portion of the secondsurface. The portion of the first surface may be bonded to the portionof the second surface. The softening strip may be bonded to a secondportion of the first surface of the spacecraft structure and to a secondportion of the second surface of the second structure, wherein thebonding steps may be performed by curing the first structure, the secondstructure, and the softening strip.

The features, functions, and advantages can be achieved independently invarious embodiments of the present disclosure or may be combined in yetother embodiments in which further details can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the advantageousembodiments are set forth in the appended claims. The advantageousembodiments, however, as well as a preferred mode of use, furtherobjectives and advantages thereof, will best be understood by referenceto the following detailed description of an advantageous embodiment ofthe present disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a diagram illustrating a spacecraft manufacturing and servicemethod in which an advantageous embodiment may be implemented;

FIG. 2 is a diagram of a spacecraft in which advantageous embodimentsmay be implemented;

FIG. 3 is a diagram illustrating a spacecraft in accordance with anadvantageous embodiment;

FIG. 4 is a diagram of a spacecraft in accordance with an advantageousembodiment;

FIG. 5 is a diagram of a cross-sectional portion of a launch vehicle inaccordance with an advantageous embodiment;

FIG. 6 is a diagram of a joint in accordance with an advantageousembodiment;

FIG. 7 is a diagram illustrating a softening strip in accordance with anadvantageous embodiment;

FIG. 8 is a diagram illustrating a three-dimensional preform inaccordance with an advantageous embodiment;

FIG. 9 is a diagram illustrating a softening strip in accordance with anadvantageous embodiment;

FIG. 10 is a flowchart of a process for creating a Y-joint in astructure in accordance with an advantageous embodiment;

FIG. 11 is a flowchart of a process for creating a Y-joint for a tank inaccordance with an advantageous embodiment;

FIG. 12 is a flowchart of a process for creating a Y-joint between acomposite cryogenic tank and a composite skirt in accordance with anadvantageous embodiment; and

FIG. 13 is a flowchart of a process for creating a softening strip inaccordance with an advantageous embodiment.

DETAILED DESCRIPTION

Referring more particularly to the drawings, embodiments of thedisclosure may be described in the context of the spacecraftmanufacturing and service method 100 as shown in FIG. 1 and spacecraft200 as shown in FIG. 2. Turning first to FIG. 1, a diagram illustratinga spacecraft manufacturing and service method is depicted in accordancewith an advantageous embodiment.

During pre-production, exemplary spacecraft manufacturing and servicemethod 100 may include specification and design 102 of spacecraft 200 inFIG. 2 and material procurement 104. During production, component andsubassembly manufacturing 106 and system integration 108 of spacecraft200 in FIG. 2 takes place. Thereafter, spacecraft 200 in FIG. 2 may gothrough certification and delivery 110 in order to be placed in service112. While in service by a customer, spacecraft 200 in FIG. 2 isscheduled for routine maintenance and service 114, which may includemodification, reconfiguration, refurbishment, and other maintenance orservice.

Each of the processes of spacecraft manufacturing and service method 100may be performed or carried out by a system integrator, a third party,and/or an operator. In these examples, the operator may be a customer.For the purposes of this description, a system integrator may include,without limitation, any number of spacecraft manufacturers andmajor-system subcontractors; a third party may include, for example,without limitation, any number of venders, subcontractors, andsuppliers; and an operator may be a country, leasing company, militaryentity, service organization, and so on.

With reference now to FIG. 2, a diagram of a spacecraft is depicted inwhich advantageous embodiments may be implemented. In this example,spacecraft 200 may be produced by spacecraft manufacturing and servicemethod 100 in FIG. 1. Spacecraft 200 may include structure 202 with aplurality of systems 204 and interior 206. Examples of systems 204include, for example, without limitation, one or more of propulsionsystem 208, electrical system 210, hydraulic system 212, andenvironmental system 214. Any number of systems may be included.Further, in some implementations some of the systems may not be needed.For example, when spacecraft 200 takes the form of a launch vehicle,environmental system 214 may be unnecessary.

Apparatus and methods embodied herein may be employed during any one ofstages of spacecraft manufacturing and service method 100 in FIG. 1. Forexample, components or subassemblies produced in component andsubassembly manufacturing 106 in FIG. 1 may be fabricated ormanufactured in a manner similar to components or subassemblies producedwhile spacecraft 200 is in service 112 in FIG. 1.

Also, one or more apparatus embodiments, method embodiments, or acombination thereof may be utilized during production stages, such ascomponent and subassembly manufacturing 106 and system integration 108in FIG. 1 as an example, without limitation. These embodiments maysubstantially expedite the assembly of or reduce the cost of spacecraft200.

In these examples, advantageous embodiments may be implemented to attachcomponents in propulsion system 208 to structure 202 in spacecraft 200.The different advantageous embodiments may be applied to attach anystructure within spacecraft 200 to any other structure in otherimplementations.

The different advantageous embodiments take into account that it may bedesirable to increase the strength of currently available joints betweenthe structure of a spacecraft and a tank containing a propellant abovecurrent levels with respect to various forces. Forces on the joint mayinclude, for example, without limitation, shear forces at the jointbetween the structure of a spacecraft and a tank. Current tanks may bemetal tanks that typically use welded joints. In these examples, thejoints may take the form of Y-joints. Composite tanks may be employed toprovide weight savings over metal tanks. With these types of compositetanks, however, Y-joints with increased strength may not be possiblewith currently available Y-joint designs and techniques.

The different advantageous embodiments may use a softening strip toextend the strength of a bonded Y-joint between two structures. Forexample, without limitation, currently available large solid rocketmotors may use a rubber softening strip in the Y-joints. The differentadvantageous embodiments take into account and recognize that adifficulty may be present in finding an appropriate material for asoftening strip that may be usable for the different operatingtemperatures. With cryogenic tanks, these temperatures may be very lowtemperatures, such as, for example, without limitation, below −238degrees Fahrenheit. These temperatures may also be referred to ascryogenic temperatures.

The different advantageous embodiments recognize that an appropriatematerial may remain soft at the different operating temperatures. Rubbermaterials, as currently used in solid rocket motors, may not be usableat cryogenic temperatures because these materials may become too stiff.The material needed may remain soft relative to the adherends while atcryogenic temperatures. An adherend may be a body attached to anotherbody by the means of adhesive substance.

The different advantageous embodiments also take into account andrecognize that a softening strip may have a coefficient of thermalexpansion that may be matched or within some threshold limits of theadherends to prevent joint failure from thermally induced stresses.

Thus, the different advantageous embodiments provide a method andapparatus for reinforcing or forming joints, such as, for example,without limitation, Y-joints. In the different advantageous embodiments,a three-dimensional preform may be present in which a plastic matrix isimpregnated into the three-dimensional preform to form a softeningstrip. This softening strip may be capable of remaining flexible at atemperature at which a material has a liquid form and in which thematerial has a gas form at an ambient temperature. In other words, thedifferent advantageous embodiments may be used at temperatures at whicha gas has a liquid state or form. As an example, without limitation, thesoftening strip may be capable of remaining flexible at cryogenictemperatures. A cryogenic temperature may be a temperature at whichconstituents of air may liquefy. A cryogenic temperature may be, forexample, temperatures below around −150 degrees centigrade.

With reference now to FIG. 3, a diagram illustrating a spacecraft isdepicted in accordance with an advantageous embodiment. Spacecraft 300is an example of spacecraft 200 in FIG. 2. In this example, spacecraft300 may include structure 302 and structure 304. Structure 302 may bethe structure of the spacecraft providing a frame or other support forother components in spacecraft 300. Structure 304, in this example, maybe, for example, without limitation, tank 306. Of course, structure 304may be any other component that may be located within spacecraft 300that is to be attached to structure 302.

Surface 308 of structure 302 may be attached to surface 310 of structure304 to form joint 312. In these examples, joint 312 may take the form ofY-joint 314. The attachment, in these examples, may be performed bybonding surface 308 to surface 310. Bonding may be a process forfastening components or structures to each other. In these examples,bonding may be performed in a number of different ways. Bonding mayinclude, for example, without limitation, using adhesives, welding,fasteners, curing processes, or some other suitable process.

The different advantageous embodiments may be applied to any type ofstructural joint such as, for example, without limitation, a joint inwhich loads peak near the shortest, stiffest load paths into a joint andthen fall off to longer, softer load paths. Any extensive shear carryinglap joint may tend to transfer load more rapidly at the edges of thejoint than away from the edges. This situation may be a consequence ofload distribution through a structure being proportional to the relativestiffness of the available load paths.

Given the same cross-sectional areas, short load paths may be stifferthan long load paths. This situation may create the familiar phenomenonof shear peaking at joint ends. If an additional soft load path can becreated outside the start of such a joint, part of the total load can betransferred through this additional path, lowering the load to becarried by the original joint and reducing shear peaking. This may be afeature of the softening strip.

A refinement in some joints may be to taper the softening strip so thatthe stiffness of the secondary load paths may be continually increasedas the loads approach original joint. This feature may result in a moreuniform transfer of shear across the joint at a consistent low level.This transfer may occur instead of the abruptly peaking shearcharacteristic of the simple lap joint or the two, more moderate, shearpeaks that may be created by using a uniform thickness softening strip.

In this depicted example, strip 316 may be attached to surface 308 andsurface 310 in and/or near joint 312. Strip 316 may act as a softeningstrip to increase the strength of joint 312 with respect to forces thatmay be applied to joint 312. These forces may include, for example,without limitation, shear forces on structures 302 and 304. In theseexamples, a shear force may be a force applied to structures 302 and 304in the direction of line 318.

In the different advantageous embodiments, strip 316 may includethree-dimensional preform 320 and plastic matrix 322. Three-dimensionalpreform 320 may be a three-dimensional structure and may take the formof three-dimensional fabric 324. This fabric may be, for example,without limitation, a fabric comprised of at least one of woven fibers,braided fibers, stacked fabric layers, and/or some other suitablematerial. When fibers are used, these fibers may include, for example,without limitation, graphite fibers, glass fibers, aramid fibers, metalfibers, or any other suitable structure fiber.

As used herein, the phrase “at least one of”, when used with a list ofitems, means that different combinations of one or more of the items maybe used, and only one of each item in the list may be needed. Forexample, “at least one of item A, item B, and item C” may include, forexample, without limitation, item A, or item A and item B. This examplealso may include item A, item B, and item C, or item B and item C.

Plastic matrix 322 may be comprised of any plastic material that mayretain or provide flexibility at operating temperatures for tank 306.Operating temperatures for tank 306 may be temperatures at or nearcryogenic temperatures. In these examples, a plastic may be anysynthetic or semi-synthetic polymerization material or product.Polymerization may be a process of reacting monomer molecules togetherin a chemical reaction to form linear chains or a three-dimensionalnetwork of polymer chains. Examples of types of plastics that may beused include, for example, without limitation, fluorocarbons andurethanes. One example of a fluorocarbon that may be used is a Teflon®polymer. Teflon® is a registered trademark of DuPont Company.

In this manner, tank 306 may be attached to structure 302 with joint 312being formed in which strip 316 may be present. The use of strip 316 mayprovide additional reinforcement to add strength to resist variousforces, such as, for example, without limitation, shear forces that maybe applied to structure 304 and tank 306.

The illustration of spacecraft 300 in FIG. 3 is provided to illustrateone implementation for various advantageous embodiments. Thisillustration is not meant to imply architectural or physical limitationsto a manner in which the different advantageous embodiments may beimplemented. For example, structure 302 and structure 304 may bestructures within another vehicle other than spacecraft 300. Forexample, the structures may be ones present in an aircraft. In otheradvantageous embodiments, spacecraft 300 may take the form of aspacecraft such as, for example, without limitation, a shuttle, a launchvehicle, a combination of a space shuttle and launch vehicle, or someother suitable vehicle capable of space travel.

With reference now to FIG. 4, a diagram of a spacecraft is depicted inaccordance with an advantageous embodiment. In this example, launchvehicle 400 is an example of one implementation of spacecraft 300 inFIG. 3. Launch vehicle 400 has forward section 402 and aft section 404.Launch vehicle 400 may have longitudinal axis 406. Launch vehicle 400 isan example of a spacecraft in which Y-joint 314 in FIG. 3 may beimplemented. Section 408 may be an example of a portion of launchvehicle 400 in which Y-joint 314 may be found and/or implemented.

With reference now to FIG. 5, a diagram of a cross-sectional portion ofa launch vehicle is depicted in accordance with an advantageousembodiment. In this diagram, a cross-section of section 408 of launchvehicle 400 along longitudinal axis 406 is depicted.

In this example, arrow 501 points towards the forward part of launchvehicle 400, while arrow 503 points to the aft portion of launch vehicle400.

In this illustrative example, tank 504 in launch vehicle 400 may includedome 506, dome 508, and wall 510. Wall 510 may be a cylindrical wall inthese examples. Wall 510 may be attached to forward skirt 512 and aftskirt 514. Wall 510 may be located between forward skirt 512 and aftskirt 514. In this example, wall 510 may be a pressurized portion oftank 504, while forward skirt 512 and aft skirt 514 may be located atunpressurized portions of tank 504.

Forward skirt 512 and aft skirt 514 are examples of structures to whichtank 504 may be attached. In these examples, these different structuresmay be cylindrical in form of which only a cross-section may be seen inFIG. 5. In this depicted example, Y-joint 520 and Y-joint 522 may bepresent. Y-joint 520 may be formed from the intersection of dome 506,wall 510 and forward skirt 512. Y-joint 522 may be formed at theintersection of dome 508, wall 510, and aft skirt 514. In theseexamples, Y-joint 520 and Y-joint 522 may be continuous incircumferential direction around axis 524.

In these examples, tank 504 may take the form of a composite cryogenictank. Tank 504 may hold a propellant, such as, for example, withoutlimitation, liquid hydrogen and/or liquid oxygen. In this example, tank504 may hold liquid hydrogen. In this example, tank 504 may be greaterthan around 16 feet in diameter. A more detailed illustration of Y-joint520 in section 530 is shown in FIG. 6 below.

Turning now to FIG. 6, a diagram of a joint is depicted in accordancewith an advantageous embodiment. As illustrated in section 530, forwardskirt 512 may include outboard skin 600, core 602, and inboard skin 604.In this example, outboard skin 600 and inboard skin 604 may be facesheets with core 602 being located between those face sheets. Core 602may be a low density structure element. Core 602 may be used, amongstother uses, to transmit loads between outboard skin 600 and inboard skin604. Core 602 may take various forms. For example, core 602 may haveflutes, honeycombs, or other suitable forms. In these examples, a flutedstructure may be a desired structure for core 602.

In this illustrative example, Y-joint 520 may be formed at theintersection of dome 506, wall 510, and forward skirt 512. Y-joint 520may have softening strip 606 located in or near Y-joint 520. Softeningstrip 606 may be bonded to inboard skin 604 and tank 504.

In the different advantageous embodiments, softening strip 606 may beco-bonded to inboard skin 604 and tank 504 along with the bonding ofinboard skin 604 to tank 504. In other words, these different componentsmay be co-bonded to each other at the same time. The co-bonding, inthese examples, may take the form of one or more curing processes inwhich the composite components in softening strip 606, inboard skin 604,and tank 504 may be cured to bond these components to each other.

The illustration of tank 504 is presented for purposes of depicting onemanner in which different advantageous embodiments may be implemented.The softening strip 606, in the different advantageous embodiments, maybe applied to other tank and/or skirt geometries. For example, althoughtank 504 is shown with a cylindrical wall, other tank configurations maybe employed. As an example, a conical wall for wall 510 may be used.Also, other tanks may not be readily symmetric. Regardless of thestructure or shape of tank 504, a Y-joint may be used.

With reference now to FIG. 7, a diagram illustrating a softening stripis depicted in accordance with an advantageous embodiment. In theillustrated example, softening strip 606 is shown in a perspective view.The illustration of softening strip 606 in this example is only aportion of softening strip 606. Softening strip 606 may be circular inshape to follow the perimeter of wall 510. The dimensions of softeningstrip 606 may vary depending on the particular implementation. In thisexample, end 700 of softening strip 606 may have a thickness of around0.2 inches in section 708. In these examples, end 702 may have athickness that is as thin as practicable to prevent introduction of astress riser in Y-joint 520. In these illustrative examples, a thicknessof end 702 may be as thin as practical, based on the ease ofmanufacturing and/or handling.

With respect to the use of softening strip 606 in Y-joint 520, softeningstrip 606 may have a length of around 4 inches from end 700 to end 702as shown in section 710. Further, softening strip 606 may extend aroundthe full circumference of tank 504 in these examples. Other dimensionsof softening strip 606 may vary depending on the particularimplementation. Of course, softening strip 606 may have any dimensionsneeded to be placed within a Y-section or other space in a joint betweenstructures. Although a constant and/or uniform cross-section is shownfor softening strip 606, the cross-section may vary depending on theconfiguration of the joint.

Softening strip 606 may be shaped in a manner to fit within Y-joint 520as shown in FIG. 6. In this example, softening strip 606 may have awedge shape in which softening strip 606 may be thicker at end 700 thanat end 702.

Faying surface 704 on side 705 of softening strip 606 may be bonded toinboard skin 604 in FIG. 6 and faying surface 706 on side 707 ofsoftening strip 606 may be bonded to tank 504 in FIG. 5. Softening strip606 may be co-cured with the other components forming Y-joint 520 asshown in FIG. 6.

With reference now to FIG. 8, a diagram of a three-dimensional preformis depicted in accordance with an advantageous embodiment. Preform 800is an example of three-dimensional preform 320 in FIG. 3.

In the illustrative example, preform 800 may be a woventhree-dimensional graphite fiber preform. Of course, other types ofmaterials and other types of structures may be used for preform 800.Other examples of materials that may be used in preform 800 include, forexample, without limitation, fiberglass fibers, boron fibers, aramidfibers, polyethylene fibers, and other suitable materials. Preform 800may be formed from braided fibers or stacked or layered fabric. Thestack of fabric materials may then be held together using z-pinning inwhich pins may be inserted and pushed into the stack of fabric in anautoclave with pressure. As another example, fabric materials forpreform 800 may be stitched to each other in the third dimension asindicated by arrow 802.

With reference now to FIG. 9, a diagram illustrating a softening stripis depicted in accordance with an advantageous embodiment. In thisexample, preform 800 may have been infused with plastic matrix 900. Theplastic material within plastic matrix 900 may be placed into preform800 using any method or process suitable for placing plastic matrix 900into preform 800.

In these examples, plastic matrix 900 may be any plastic materialcapable of maintaining flexibility at temperatures in which a materialmay be in a liquid form when that material normally may be in a gas format ambient temperatures. In these examples, ambient temperature may bethe temperature of the environment around the spacecraft. In otherwords, the ambient temperature may be the temperature in the air aroundthe spacecraft when the spacecraft is on the ground.

This combination of preform 800 and plastic matrix 900 forms softeningstrip 902. Softening strip 902 may then be machined, cut, and/or formedin some other suitable fashion in a shape, such as softening strip 606in FIG. 6 for use in a Y-joint.

Preform 800 may restrain softening strip 902 against excessive thermalshrinkage that may be associated with high coefficients of expansion.These high coefficients of expansion may be typical of softening stripmatrix materials, such as those for plastic matrix 900, which may remainflexible at cryogenic temperatures. Excessive shrinkage in any directionmay create thermally induced stresses in the bond lines betweensoftening strip 902 and any adjacent structure. These types of stressesmay weaken the joint. As a result, softening strip 902 may beconstructed with preform 800 to provide three-dimensional reinforcement,rather than two-dimensional reinforcement.

With reference next to FIG. 10, a flowchart of a process for creating aY-joint in a structure is depicted in accordance with an advantageousembodiment. The process illustrated in FIG. 10 may be used to create ajoint between a first structure and a second structure.

The process begins by placing a softening strip into a joint region fora first structure (operation 1000). The softening strip is capable ofremaining flexible at a temperature at which the gas has a liquid form.The process then lays up and/or positions a second structure (operation1002). Thereafter, the softening strip is bonded to the first structureand the second structure to form the joint (operation 1004), with theprocess terminating thereafter. This bonding may be performed usingvarious mechanisms, including co-curing, curing, or other suitablebonding techniques.

With reference to FIG. 11, a flowchart of a process for creating aY-joint for a tank is depicted in accordance with an advantageousembodiment. The process illustrated in FIG. 11 may be a detailed exampleof one implementation of the processes in FIG. 10. The processillustrated in FIG. 11 may be used to create a Y-joint between acomposite cryogenic tank and a composite skirt.

The process may begin by laying the dome and wall of the tank (operation1100). Thereafter, a softening strip may be placed into the joint region(operation 1102). In operation 1002, the softening strip may be placedinto the region with an adhesive film.

Next, a skirt structure is laid up (operation 1104). The skirt structuremay be laid up over the area where the dome and wall may be locatedalong with the softening strip. The components are then cured (operation1106), with the process terminating thereafter. The curing step may beperformed using heat and/or pressure. This curing may be performed usingan autoclave or some other suitable oven. In other advantageousembodiments, these components may be cured using an electron beam. Theresult of curing operation 1106 may be a joint similar to joint 520 inFIG. 5.

With reference now to FIG. 12, another flowchart of a process forcreating a Y-joint between a composite cryogenic tank and a compositeskirt is depicted in accordance with an advantageous embodiment.

The process may begin by laying up the dome and inboard face sheet ofthe wall of the tank on a tool and curing the lay up (operation 1200).The dome may be, for example, without limitation, dome 506, and the facesheet of the tank wall may be for a wall such as, for example, withoutlimitation, wall 510 in FIG. 5. In these examples, the tool may be amold for the shape of the different composite components.

The process may place a film adhesive on faying surfaces 704 and 706 forthe softening strip (operation 1202). The softening strip in operation1202 may be a softening strip such as, for example, without limitation,softening strip 606 in FIG. 7. In these examples, faying surfaces 704and 706 are the surfaces of a structure that are to be bonded to anotherstructure. In other words, when two structures are bonded to each other,the surfaces that contact each other to form the joint may be referredto as the faying surfaces.

The process may then position the softening strip against the adhesiveon the dome and inboard face sheet for the wall of the tank (operation1204). The process may then cure the adhesive joining softening strip tothe tank using local heater blankets (operation 1206). After operation1206, the softening strip may be bonded to the tank wall. The softeningstrip may then be trimmed into a final shape in place on the tank wall(operation 1208). The process may then position a skirt tool and attachthe skirt tool to the tank tool (operation 1210).

The process may then place film adhesive on faying surfaces 704 and 706for the inboard wall of skirt 512 (operation 1212). The process may thenlay up the inner wall of the skirt and cure the component in place(operation 1214).

Next, adhesive may be placed on faying surfaces 704 and 706 for skirt512 and core 602 (operation 1216). Core 602 may be the structuralelements located between two face sheets for the wall of the structure.The process may then lay up the wall sandwich core and outboard facesheet and cure the components in place (operation 1218) with the processterminating thereafter. In operation 1218, the outboard face sheet maybe, for example, outboard skin 600 in FIG. 6.

With reference now to FIG. 13, a flowchart of a process for creating asoftening strip is depicted in accordance with an advantageousembodiment. The process illustrated in FIG. 13 may be used to create asoftening strip for use in the operations in FIGS. 11 and 12.

The process may begin by forming a three-dimensional preform (operation1300). In these examples, operation 1300 may be performed using, forexample, without limitation, braiding fibers, weaving fibers, stackingmaterials and performing z-pinning, or some other suitable process.

Next, a plastic matrix may be infused into the three-dimensional preform(operation 1302). In these examples, the plastic matrix may be a Teflon®polymer. With this type of polymer, a dry powder may be placed onto thethree-dimensional preform and worked into the preform. For example, theinfusion in operation 1302 may involve vibrating the preform and thenapplying heat and pressure to perform the infusion into the preform. Theheat and pressure in operation 1302 may be performed using an autoclave.

The preform may be shaped (operation 1304). In operation 1304, thepreform may be machined, cut, or shaped using some other suitableprocess into a shape for use in a Y-joint. The process may etch thesurfaces, faying surface 704 and faying surface 706 of the softeningstrip (operation 1306). This etching may be an acid etch. Tertra-Etch®fluorocarbon etchant is an example of a commercially available etchantused to prepare fluorocarbon surfaces for bonding. Tertra-Etch®fluorocarbon etchant may be available from W. L. Gore and Associates,Inc., and Tertra-Etch® is a trademark of W. L. Gore and Associates, Inc.This etching may be used to allow an adhesive film to be attached to thesurface of the preform. The process may then attach an adhesive film toeach side, side 708 and side 707, of the softening strip (operation1308), with the process terminating thereafter. The adhesive strips mayallow the softening strip to be attached and cured to the skirt and thedome of the tank during the curing process.

The different operations shown in the flowcharts may not be inclusive ofall of the different steps that may be performed for the differentadvantageous embodiments. For example, other operations such as, forexample, without limitation, preparation for bonding, shimming, caulplating, and other suitable operations may be performed during thedifferent processes illustrated above. Also, in some advantageousembodiments, some of the operations may be performed concurrently or indifferent orders, depending upon the particular implementation.

Thus, the different advantageous embodiments provide a method andapparatus for increasing the strength of a joint between differentstructures. The different advantageous embodiments may include athree-dimensional preform with a plastic matrix impregnated or infusedinto the three-dimensional preform to form a softening strip. Thissoftening strip is capable of remaining flexible at a temperature inwhich a material having a gas form at an ambient temperature has aliquid form.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art.

Although the different illustrative examples show the use of a softeningstrip in a Y-joint for a spacecraft, this softening strip may be used inobjects other than spacecraft. For example, without limitation, asoftening strip may be used in a joint for structures in a submarine, anaircraft, a building, a dam, a manufacturing facility, a power plant, atank, a car, or some other suitable object.

Further, different advantageous embodiments may provide differentadvantages as compared to other advantageous embodiments. The embodimentor embodiments selected are chosen and described in order to bestexplain the principles of the embodiments, the practical application,and to enable others of ordinary skill in the art to understand thedisclosure for various embodiments with various modifications as aresuited to the particular use contemplated.

1. An apparatus comprising: a three-dimensional preform; and a plasticmatrix impregnated in the three-dimensional preform to form a softeningstrip that is capable of remaining flexible at a temperature at which agas has a liquid form.
 2. The apparatus of claim 1 further comprising: afirst structure having a first side; and a second structure having afirst side bonded to the first side of the first structure, wherein thesoftening strip is bonded to a portion of the first side of the firststructure and a portion of the first side of the second structure toform a joint.
 3. The apparatus of claim 2, wherein the joint is aY-joint.
 4. The apparatus of claim 2, wherein the first structure is astructure for one of a launch vehicle, a spacecraft, and an aircraft. 5.The apparatus of claim 4, wherein the second structure is a tank capableof holding a liquid propellant.
 6. The apparatus of claim 2, wherein thefirst structure comprises a material selected from one of a metal, ametal alloy, and a composite material.
 7. The apparatus of claim 2,wherein the second structure comprises a material selected from one of ametal, a metal alloy, and a composite material.
 8. The apparatus ofclaim 1, wherein the three-dimensional preform is a three-dimensionalfabric.
 9. The apparatus of claim 8, wherein the three-dimensionalfabric comprises at least one of weaved fibers, braided fibers, andstacked fabric layers.
 10. The apparatus of claim 9, wherein the fibersmatrix are graphite fibers.
 11. The apparatus of claim 1, wherein theplastic is selected from at least one of a fluorocarbon and a urethane.12. A method for forming a joint, the method comprising: placing asoftening strip into a joint region for a first structure, wherein thesoftening strip is capable of remaining flexible at a temperature atwhich a gas has a liquid form; laying up a second structure; and bondingthe softening strip to the first structure and to the second structureto form the joint.
 13. The method of claim 12, wherein the bonding stepcomprises: co-curing the first structure, the second structure, and thesoftening strip.
 14. The method of claim 12, wherein the softening stripcomprises: a three-dimensional preform; and a plastic matrix impregnatedin the three-dimensional preform to form the softening strip.
 15. Themethod of claim 12, wherein the first structure is a spacecraftstructure and the second structure is a cryogenic tank.
 16. The methodof claim 12 further comprising: forming the three-dimensional preform;infusing a plastic material into the three-dimensional preform to formthe softening strip; attaching a first adhesive layer to a first side ofthe softening strip; and attaching a second adhesive layer to a secondside of the softening strip.
 17. The method of claim 12, wherein thethree-dimensional preform is a three-dimensional fabric.
 18. The methodof claim 17, wherein the three-dimensional fabric is comprises of atleast one of weaved fibers, braided fibers, and stacked fabric layers.19. The method of claim 18, wherein the fibers are selected from one ofgraphite fibers, glass fibers, and aramid fibers.
 20. The method ofclaim 13, wherein the plastic matrix is selected from at least one of afluorocarbon and a urethane.
 21. A method for manufacturing a softeningstrip, the method comprising: forming a three-dimensional preform; andinfusing a plastic material into the three-dimensional preform to formthe softening strip that is flexible at a temperature at which a gas hasa liquid form.
 22. The method of claim 21 further comprising: attachinga first adhesive layer to a first side of the softening strip; andattaching a second adhesive layer to a second side of the softeningstrip.
 23. The method of claim 21, wherein the three-dimensional preformis a three-dimensional fabric.
 24. A launch vehicle comprising: astructure having a first side, wherein the structure is comprised of amaterial selected from at least of a metal, a metal alloy, and acomposite material; a tank capable of holding a liquid propellant andhaving a first side bonded to the first side of the structure, whereinthe tank is comprised of a material selected from one of a metal, ametal alloy, and a composite material; and a softening strip having athree-dimensional preform and a plastic matrix impregnated in thethree-dimensional preform to form the softening strip that is capable ofremaining flexible at a temperature at which a gas has a liquid form,wherein the three-dimensional preform is a three-dimensional fabriccomprising at least one of weaved fibers, braided fibers, and stackedfabric layers in which the fibers are selected from one of graphitefibers, glass fibers, and aramid fibers, wherein the plastic matrix isselected from at least one of a fluorocarbon and a urethane, and whereinthe softening strip is bonded to the first side of the first structureand the first side of the tank to form a Y-joint.
 25. A method forforming a joint in a spacecraft, the method comprising: forming athree-dimensional fabric preform; infusing a plastic material into thethree-dimensional fabric preform to form a plastic matrix to form asoftening strip, wherein the softening strip is capable of remainingflexible at a temperature at which a gas has a liquid form, wherein thethree-dimensional fabric preform is comprised of at least one of weavedfibers, braided fibers, and stacked fabric layers in which the fibersare graphite fibers, and wherein the plastic material is selected fromat least one of a fluorocarbon and a urethane; attaching a firstadhesive layer to a first side of the softening strip; attaching asecond adhesive layer to a second side of the softening strip; placingthe softening strip into a joint region for a first structure of aspacecraft, wherein the softening strip is capable of remaining flexibleat a temperature at which a gas has a liquid form ; laying up a secondstructure for the spacecraft; and co-curing the softening strip to thefirst structure and to the second structure to form the joint.